Dome-forming volcanoes are among the most hazardous volcanoes on Earth. Magmatic outgassing can be hindered if the permeability of a lava dome is reduced, promoting pore pressure augmentation and ...explosive behaviour. Laboratory data show that acid-sulphate alteration, common to volcanoes worldwide, can reduce the permeability on the sample lengthscale by up to four orders of magnitude and is the result of pore- and microfracture-filling mineral precipitation. Calculations using these data demonstrate that intense alteration can reduce the equivalent permeability of a dome by two orders of magnitude, which we show using numerical modelling to be sufficient to increase pore pressure. The fragmentation criterion shows that the predicted pore pressure increase is capable of fragmenting the majority of dome-forming materials, thus promoting explosive volcanism. It is crucial that hydrothermal alteration, which develops over months to years, is monitored at dome-forming volcanoes and is incorporated into real-time hazard assessments.
The Middle Miocene Upper Freshwater Molasse sediments represent the last cycle of clastic sedimentation during the evolution of the North Alpine Foreland Basin. They are characterized by small-scale ...lateral and temporal facies changes that make intra-basin stratigraphic correlations at regional scale difficult. This study provides new U–Pb zircon ages as well as revised
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Ar data of volcanic ash horizons in the Upper Freshwater Molasse sediments from southern Germany and Switzerland. In a first and preliminary attempt, we propose their possible correlation to other European tephra deposits. The U–Pb zircon data of one Swiss (Bischofszell) and seven southern German (Zahling, Hachelstuhl, Laimering, Unterneul, Krumbad, Ponholz) tuff horizons indicate eruption ages between roughly 13.0 and 15.5 Ma. The stratigraphic position of the Unterneul and Laimering tuffs, bracketing the ejecta of the Ries impact (Brockhorizon), suggests that the Ries impact occurred between 14.93 and 15.00 Ma, thus assigning the event to the reversed chron C5Bn1r (15.032–14.870 Ma) which is in accordance with paleomagnetic evidence. We combine our data with published ages of tuff horizons from Italy, Switzerland, Bavaria, Styria, Hungary, and Romania to derive a preliminary tephrochronological scheme for the Middle Miocene in Central Europe in the age window from 13.2 to 15.5 Ma. The scheme is based on the current state of knowledge that the Carpathian–Pannonian volcanic field was the only area in the region producing explosive calc-alkaline felsic volcanism. This preliminary scheme will require verification by more high-quality ages complemented by isotopic, geochemical and paleomagnetic data.
•Hydraulic properties of fractured Buntsandstein sandstone are quantified in the laboratory.•Bedding and sealed fractures affect the permeability anisotropy of the Buntsandstein.•A model for the ...growth rate of barite crystals with temperature is proposed.•Fracture permeability may be time dependant within the lifetime of a geothermal site.
In geothermal reservoirs, fluid circulation is greatly dependent on the geometry, density, and hydraulic properties of fractures. The Soultz-sous-Forêts geothermal site located in the Upper Rhine Graben in Alsace, France, consists of a granitic basement overlain by a 1.4km-thick sedimentary succession. Core analysis and borehole wall imagery collected from reconnaissance well EPS1, drilled vertically to a depth of 2230m, revealed an extensive fracture network throughout the granite and overlying sediments, including both open fractures and fractures sealed through mineral precipitation (primarily quartz, illite, chlorite, calcite, dolomite, barite, pyrite and galena). Here we present a combined experimental and modelling study that aims to provide insights into the permeability anisotropy in the Triassic Buntsandstein sandstone (1–1.4km depth) and the impact of mineral precipitation. We targeted borehole samples that best represented the variability of fractures within the Buntsandstein. Forty cylindrical samples (40mm in length and 20mm in diameter) were prepared from the chosen borehole samples such that they contained sealed or partially-sealed fractures either parallel or perpendicular to their axis. We also prepared samples of the intact host rock. These samples were then subject to porosity and permeability measurements, and thin sections were made for Scanning Electron Microscopy (SEM) to characterise the nature of the fractures and the precipitated minerals. Permeability measurements of the Buntsandstein host rock yielded values ranging from 10−15m2 to less than 10−18m2. SEM and X-ray powder diffraction analyses suggest that prevalent pore-filling illitic clays can explain the low permeability of the sandstone host rock. Additionally, we found that the permeability of fractures depends on the nature of the filling and the extent of sealing, with barite providing the most effective precipitate. Taking into account the geothermal fluid composition at Soultz-sous-Forêts, we employ a kinetic model for the barite crystal growth rate with temperature to provide an estimate for the time scale over which open fractures can seal through barite precipitation (from months to days depending on temperature). The rate increases dramatically as the temperature of the geothermal brine decreases, highlighting the risk of mineral precipitation at geothermal sites, where fluid temperature fluctuates due to circulation through the reservoir rock and fluid mixing around the injection well. An improved knowledge of the time dependency of fracture permeability will provide insights into the permeability anisotropy in the Buntsandstein and may have repercussions for the geothermal exploitation and for the ongoing fluid flow modelling of the Soultz-sous-Forêts geothermal reservoir.
Volcanoes often host hydrothermal systems that alter the host rock. To understand the influence of alteration on mechanical behaviour of edifice-forming rock, we performed a series of triaxial ...deformation experiments on variably altered andesite from Mt. Ruapehu (New Zealand) under constant effective pressure. Under the imposed conditions, andesite with intermediate argillic alteration deforms in a brittle manner forming fractures. By contrast, andesite with advanced argillic alteration deforms in a ductile manner, with sample failure driven by distributed cataclastic pore collapse. We consider this the result of an increase in porosity and clay content with increasing alteration. Ancillary experiments highlight that the brittle-ductile transition occurs at lower effective pressure (i.e. at shallower depths) in andesites with advanced argillic alteration relative to unaltered andesites of comparable porosity. We conclude that advanced argillic alteration can create an anomalous shallow ductile zone, which has important implications for fluid flow and pre-eruptive seismicity.
Raman spectroscopy and laser ablation‐inductively coupled plasma‐mass spectrometry were used to characterize the chemical composition of 34 red garnet beads from Lower Nubian sites, dated between ...about 3200 BCE and 600 CE. All beads from the A‐Group to the Meroitic period feature a similar calcium‐poor almandine composition (69%–78% almandine, 15%–22% pyrope, 2%–6% grossular, 3%–9% spessartine), which differs from other calcium‐poor almandine garnet types, sourced mostly from Indian deposits in Antiquity. The Nubian beads constitute a new garnet type, named “cluster I”, featuring high yttrium (180 to 1205 ppm), moderately low chromium (13–70 ppm), and high scandium (119–213 ppm) concentrations. Their compositions match with previous and two new analyses from two alluvial garnet deposits, Wadi El‐Haraz and Wadi Abu Dom, near the Fourth Cataract of the Nile in Upper Nubia, about 670 km as the crow flies from the Lower Nubian graves. Garnet trade between the Bayuda desert and Lower Nubia sites, and possibly even Egypt, flourished for almost four millennia. Northeastern Africa is the cradle for the oldest use of a gemstone that is harder than quartz—the red almandine garnet.
A Post‐Meroitic bead, the youngest in the assembly, displays an unusual faceting, a diamond tipped drill hole, excellent polish, distinct short‐ and long‐prismatic colorless mineral inclusions, and a calcium‐ and manganese‐poor pyrope composition. This suggests that it was not of a local, Nubian, production, but imported, most probably from a South Asian site.
Phreatic eruptions are possibly the most dramatic surface expressions of hydrothermal activity, and they remain poorly understood. The near absence of precursory signals makes phreatic eruptions ...unpredictable with respect to both time and magnitude. The Valley of Desolation (VoD), Dominica, located close to the Boiling Lake, the second largest high-temperature volcanic crater lake in the world, hosts vigorous hydrothermal activity with hot springs, mud pools, fumaroles, and steaming ground. A phreatic or phreatomagmatic eruption from this site is considered to be the most likely scenario for future volcanic activity on Dominica. Yet there is little information regarding the trigger mechanisms and eruption processes of explosive events at this active hydrothermal center, and only a very small number of studies have investigated hydrothermal activity in the VoD. We therefore conducted two field campaigns in the VoD to map hydrothermal activity and its surficial phenomena. We also investigated alteration processes and their effects on degassing and phreatic eruption processes. We collected in situ petrophysical properties of clay-rich unconsolidated samples, and together with consolidated rock samples, we investigated the range of supergene and hydrothermal alteration in the laboratory. In addition, we performed rapid decompression experiments on unconsolidated soil samples. Our results show that alteration leads to an increasing abundance of clay minerals and a decrease in both strength and permeability of the rocks. In the immediate vicinity of degassing acid-sulfate fluids, advanced argillic alteration yields a mineral zoning which is influenced by meteoric water. The water-saturated basal zone is dominated by kaolinite run 0whereas alunite formation is favored at and above the groundwater table where atmospheric oxidation of H
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S to H
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SO
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occurs (e.g., steam-heated alteration). Alteration effects may in turn inhibit degassing at the surface, increasing the potential for pressurization in the subsurface and thus lead to phreatic eruptions. Rapid decompression experiments, together with ballistic trajectory calculations, constrain estimates of the conditions prior to the 1997 small-scale phreatic event in the VoD. The results presented here may serve as a contribution to the understanding of the hazard potential of ongoing hydrothermal activity within the VoD. On a broader perspective, our results will help evaluate hydrothermal activity in similar areas worldwide which might also have the potential for phreatic eruptions, for instance Poas (Costa Rica) or Tongariro and Waimangu (New Zealand).
Volcanoes are unstable structures that deform laterally and frequently experience mass wasting events. Hydrothermal alteration is often invoked as a mechanism that contributes significantly to ...volcano instability. We present a study that combines laboratory experiments, geophysical data, and large‐scale numerical modeling to better understand the influence of alteration on volcano stability, using La Soufrière de Guadeloupe (Eastern Caribbean) as a case study. Laboratory experiments on variably altered (advanced argillic alteration) blocks show that uniaxial compressive strength, Young's modulus, and cohesion decrease as a function of increasing alteration, but that the internal friction angle does not change systematically. Simplified volcano cross sections were prepared (a homogenous volcano, a volcano containing the alteration zone identified by a recent electrical survey, and a volcano with an artificially enlarged area of alteration) and mechanical properties were assigned to zones corresponding to unaltered and altered rock. Numerical modeling performed on these cross sections, using a hydro‐thermo‐mechanical modeling code, show (a) the importance of using upscaled values in large‐scale models and (b) that alteration significantly increases volcano deformation and collapse volume. Finally, we combined published muon tomography data with our laboratory data to create a 3D strength map, exposing a low‐strength zone beneath the southern flank of the volcano coincident with the hydrothermal system. We conclude that hydrothermal alteration decreases volcano stability and thus expedites volcano spreading and increases the likelihood of mass wasting events and associated volcanic hazards. Hydrothermal alteration, and its evolution, should therefore be monitored at active volcanoes worldwide.
Plain Language Summary
The rocks forming a volcanic edifice can be altered by circulating hydrothermal fluids. This alteration can influence the physical and mechanical properties of these rocks, which could jeopardize volcano stability. The stability of a volcanic edifice is an important consideration in volcanic hazards and risk assessments due to the potentially dire consequences of partial volcanic flank collapse. Using a combination of experimental data, geophysical data, and modeling, and La Soufrière de Guadeloupe (Eastern Caribbean, France) as a case study, we find that hydrothermal alteration decreases volcano stability and thus promotes volcano instability and associated volcanic hazards. As a result, we conclude that hydrothermal alteration, and its evolution, should be monitored at active volcanoes worldwide.
Key Points
Laboratory experiments show that hydrothermal alteration reduces the strength of volcanic rock from La Soufrière
Numerical modeling shows that hydrothermal alteration significantly increases volcano deformation and collapse volume
We provide a 3D strength map of La Soufrière that exposes a low‐strength zone coincident with the hydrothermal system
•Ediacaran basalts of the East European Craton were altered by heated meteoric water.•Cyanobacteria were active on the Ediacaran land in hydrothermal systems.•Chemical changes dominated by import of ...Mg and H2O and export of Ca.•The main alteration product is a mixture of di- and trioctahedral Fe-smectites; potentially analogous to the most common Martian clays.•Most of REE in the alteration products are contained in the dioctahedral smectite.
The Ediacaran continental flood basalts and associated tuffs were studied to identify and quantify alteration processes by means of XRD and chemistry, supplemented by Mössbauer and FTIR spectroscopies, petrography, oxygen and iron isotopes, K-Ar dating, and organic geochemistry. Two superimposed alteration processes were identified: the Ediacaran hydrothermal alteration, induced by meteoric waters, heated and put in motion by the cooling basalt, and the Caledonian and/or Variscan potassic alteration. The degree of basalt alteration was quantified using as an index the sum of primary minerals in the bulk rock. The sequence of minerals dissolved and crystallized during the hydrothermal alteration was established. The alteration resulted in the loss of Ca (dissolution of plagioclases), compensated by the gain of water and Mg (crystallization of clays), and proceeded from the edges of the basalt flows in an oxidizing environment, evidenced by the increasing amount of hematite and Fe3+/Fe2+ ratio of the bulk rock. Cyanobacteria were active in the hydrothermal system, most probably responsible for the measured negative δ56Fe values and more reducing conditions at the stage of intense alteration. Chlorophaeite (palagonite), following quartz as the earliest petrographically identifiable basalt alteration product was found to vary systematically from fully isotropic to birefringent. The chlorophaeite was identified as a mixture of Fe-montmorillonite and Fe-saponite, identical with griffithite and oxysmectites, probably preceded by a finer-grained ferrosaponite at the isotropic stage. REE content of chlorophaeite indicates basaltic volcanic glass (sideromelane) as the major source of material. REE in clays are contained mostly in the dioctahedral smectite, while in the bulk rock mostly in phosphates. The smectite characteristics and Mg enrichment are indicative of the hydrothermal basalt alteration process, which perhaps was dominant also on Mars.
Both mineral and chemical composition of tuffs vary continuously from basaltic to felsic, the latter close to the measured rhyodacite composition, dominated by quartz and feldspars. The basaltic tuffs resemble the most altered basalts but contain also abundant albite and chlorite, indicative of higher alteration temperatures, up to 220 °C. Tuff composition indicates stronger component of felsic volcanism in the trap formation than evidenced by the preserved bodies of effusive rocks.
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